1. Field of the Invention
The present invention relates to a semiconductor device using a field shield element-isolation and a method of making such a semiconductor device, and particularly to a semiconductor device using a field shield element-isolation in which heavy metal impurities contained in a silicon substrate are removed and a method of making such a semiconductor device.
2. Description of the Related Art
Recently, attention is directed to an element isolating method called a field shield method since in this method in that the potential of a parasitic transistor can be effectively cut off by fixing the field shield (gate) electrode to a ground potential or a predetermined potential and the interval between active regions can be reduced thus, this method is suitable for making a semiconductor device very small in size. An example of the method is disclosed by Wakayama et al. in "Fully Planarized 0.5 .mu.m Technologies for 16M DRAM" IEDM-88, 1988, pp. 246-249.
FIG. 8 shows a structure of a MOS transistor in which the field shield element-isolation by the method shown in the above-mentioned literature is applied. In this structure, after a field shield oxide film 22 of 50 nm thickness is formed first on a silicon substrate 21, boron ions are doped by ion implantation into an underside of the oxide film 22 under the condition of dose rate of 10.sup.12 ions/cm.sup.2 and implantation energy of 20.about.40 Kev in order to perform threshold adjustment for transistors 23 formed in active regions and parasitic MOS transistors in the element-isolation region. Thereafter, a field shield electrode 24 is formed of phosphor-doped polysilicon, at a thickness of 200 nm, a gate oxide film 25 and a gate electrode 26 of each transistor 23 are formed, and then source/drain regions 28 are formed by impurity diffusion.
By fixing the field shield electrode 24 to the ground potential GND, the transistor formed of the respective source and drain of adjacent active transistors 23 cannot turn on so that the elements, i.e. the transistors can be isolated properly from each other.
In the case where the conventional field shield method is used, however, unlike a known LOCOS (local oxidation of silicon) method, there is no layer for absorbing or gettering heavy metal impurity elements such as Fe, etc. contained in the silicon substrate and therefore no effective gettering. Accordingly, the impurity elements remain in the silicon substrate even after formation of a semiconductor device so that deterioration of transistor characteristic such as increase of junction leakage current, etc., resulting in a serious factor for reduction of the yield in manufacturing.